Best way to weekly update a MYSQL field? - mysql

Question

I want to weekly update a field in a MySQL table "Persons", with the avg of two fields of the "Tasks" table, end_date and start_date:
PERSON:
+----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| average_speed | int(11) | NO | | 0 | |
+----------------+-------------+------+-----+---------+-------+
TASKS:
+----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| person_id | int(11) | NO | | NULL | |
| start_date | date | NO | | NULL | |
| end_date | date | NO | | NULL | |
+----------------+-------------+------+-----+---------+-------+
(tables are not complete).
average_speed = AVG(task.end_date - task.start_date)
Now, the Tasks table is really big, and ** I don't want to compute the average on every task for every person every week**. (That's a solution, but I'm trying to avoid it).
What's the best way to update the average_speed?
I thought about adding two columns in the person's table:
"last_count": count of computed tasks since now for each person
"last_sum": last sum of (end_date - start_date) for each person
So that on a new update i could do something like average_speed = (last_sum+new_sum) / (last_count + new_count) where new_count is the sum of the tasks in the last week.
Is there a better solution/architecture?
EDIT:
to answer a comment, the query I would do is something like this:
SELECT
count(t.id) as last_count,
sum(TIMESTAMPDIFF(MINUTE, t.start_date, t.end_date)) as last_sum
avg(TIMESTAMPDIFF(MINUTE, t.start_date, t.end_date))
from tasks as t
where t.end_date BETWEEN DATE_SUB(CURDATE(), INTERVAL 1 WEEK) AND CURDATE()
And i can rely on a php script to get result and do some calculations

Answer 1

Having a periodic update to the table is a bad way to go for all the reasons you've listed above, and others.
If you have access to the code that writes to the Tasks table, that’s the best place to put the update. Add an Average field and calculate and set the value when you write the task end time.
If you don’t have access to the code, you can add a calculated field to the table that shows the average and let SQL figure it out during the execution of a query. This can slow queries down a little, but the data is always valid and SQL is smart enough to only calculate that value when it is needed.
A third (ugly) option is a trigger on the table that updates the value when appropriate. I’m not a fan of triggers because they hide business logic in unexpected places, but sometimes you just have to get the job done.